The widespread applications of optical films include such uses as polarizers, compensators, light enhancers, diffusers, substrates and protective layers, filters, and security devices. Of particular recent interest is the use of optical films as polarizers and as compensators with liquid crystal display (LCD) devices. Optical films used as polarizers adapt or condition the polarization of incident light to provide output light that is linearly, elliptically, or circularly polarized, for example. Optical films used as retarders and rotators condition light to provide suitable retardation effects.
Optical compensation films are used to improve the viewing angle characteristic of LCD displays, which describes the change in contrast ratio for different viewing angles. It is desirable that contrast be maintained over a wide range of viewing angles. One way to improve the viewing angle characteristic is to insert a compensator (also referred as compensation film, retardation film, or retarder) with proper optical properties between the polarizer and liquid crystal cell, such as disclosed in U.S. Pat. No. 5,583,679 (Ito et al.), U.S. Pat. No. 5,853,801 (Suga et al.), U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,978,055 (Van De Witte et al.), and U.S. Pat. No. 6,160,597 (Schadt et al.). One widely used compensation film according to U.S. Pat. No. 5,583,679 (Ito et al.) and U.S. Pat. No. 5,853,801 (Suga et al.) is based on discotic liquid crystals which exhibit negative birefringence. This film offers improved contrast over wider viewing angles; however, it suffers larger color shift for gray level images, compared to compensators made of liquid crystalline materials having positive birefringence, according to Satoh et al. (“Comparison of Nematic Hybrid and Discotic Hybrid Films as Viewing Angle Compensator for NW-TN-LCDs” SID 2000 Digest, pp. 347-349, (2000)). To achieve comparable contrast ratio while reducing color shift, one compensation film solution, for example, as disclosed by Chen et al. (“Wide Viewing Angle Photoaligned Plastic Films” SID 99 Digest, pp. 98-101 (1999)), uses a pair of liquid crystal polymer films (LCP), treated to have orthogonally crossed optical axes, disposed on each side of a liquid crystal cell.
Types of Photoalignment Methods
In processing liquid crystal compensation films, photo-alignment methods are recognized to have advantages over earlier rubbing alignment methods. Using photo-alignment, a thin photo-reactive alignment medium, typically linear photo-polymerization media (LPP), also known as photo-oriented polymer network (PPN), is applied to a substrate and is then irradiated, typically using UV light, to provide a directional alignment bias. There are a number of photo-alignment methods, based on different photoreaction processes. In general, a photo-alignment method may be one of three basic types:                (1) Isomerization, as disclosed in U.S. Pat. No. 4,974,941 (Gibbons et al.) is a reversible process using laser light irradiation in which a monomer or single molecule is aligned using cis-trans-isomerization effects;        (2) Photo-dimerization, as disclosed in U.S. Pat. No. 5,602,661 (Schadt et al.) employs photo-induced orientation and dimerization of polymer side-chains, including cross-linking; and        (3) Photo-dissociation, as taught in “Prospects of the Photo-Alignment Technique for LCD Fabrication” SID Digest 1997, pp. 311-314 (Iimura et al.) uses light to anisotropically alter an alignment medium such as polyamic acid or polyimide or copolymer comprised of amic acid and imide.        
In one promising photo-dimerization method, a liquid crystal polymer (LCP) layer is applied over an LPP layer that has been treated to provide a preferred alignment orientation. As is well know in the photoalignment art, LCP materials include cross-linkable liquid crystalline monomers, oligomers, or pre-polymers having cross-linkable groups. Depending on the intended application, the cross-linkable liquid crystal material may exhibit a nematic, smectic, or cholesteric phase. Most solutions for photo-alignment using this method direct collimated, polarized UV light, at an oblique angle, onto an alignment LPP substrate to align polymer molecules in a desired direction that provides a pretilt for a subsequently applied LCP layer containing liquid crystal structures. It has been found that, for suitable performance, only a fraction of molecules in the LPP alignment layer need to be photopolymerized. Typical photoreactive LCP media include diacrylates and diepoxides and similar cross-linkable liquid crystalline materials. LCPs may have inherent positive optical anisotropy, such as with diacrylates, or negative anisotropy and weak biaxial properties, such as with discotic liquid crystal materials.
General Criteria for Photoalignment Irradiation
A number of different photo-alignment media and techniques have been used to provide the necessary pretilt orientation for different types of liquid crystal display media. For a suitable class of LPP media, optical apparatus that provides irradiation for alignment must provide the following:                1. Sufficient exposure levels, typically in the nominal range of 10-15 mJ/cm2.        2. Narrow range of wavelengths. The exact range that is suitable for alignment irradiation depends on the particular photoreactive material. For example, UV-B (280-320 nm) is the preferred range for many types of photoreactive alignment layer. Some wavelengths are preferably rejected in order to minimize unwanted effects on alignment or undesirable temperature effects. For example, UV light is efficiently produced by a class of lamps that excite mercury or ion-doped mercury molecules. Such lamps typically generate UV-C (200-280 nm), UV-B (280-320 nm), UV-A (320-400 nm), visible light, and infrared light. For an embodiment where UV-B is chosen as the preferred spectral range, it would be desirable to limit the irradiance and total exposure on the web from other parts of the spectrum.        3. Uniform exposure dosage. Exposure dose is expressed in terms of energy per unit area. It has been found that dosage levels, alternately termed exposure levels, can provide acceptable alignment results even where dosage varies by as much as +/−50% across the irradiated surface area in some applications. However, reasonable dosage uniformity helps to obtain uniform alignment results, minimizing intensity level variations between levels at the middle of a substrate and at substrate edges.        4. Uniform direction of polarization. It does not appear to be important that the applied alignment radiation be highly polarized. However, for a class of LPP materials, best results are achieved when the exposure radiation has a highly uniform direction of polarization.        5. Oblique incident angles for pretilt. Typically, some deviation from normal incidence to the media is desirable in order to provide the necessary pretilt angle to the LPP material. For most applications, a broad range of incident angles is permissible, such as from 10 to 70 degrees, for example.        
There have been some conventional systems developed that generally meet most of requirements 1-5 above for irradiating photoreactive alignment media on a small scale. However, it can be appreciated that these requirements become particularly difficult to meet as the irradiated surface area, or exposure zone, increases. Conventional solutions are as yet poorly suited to the demands for efficiently irradiating a web-fed photoreactive substrate, where the substrate is moved past the irradiation device at production speeds and the web width exceeds 1 m. Among the problems encountered with such systems is relatively poor light efficiency. Losses from light directing and collimating components, filters, and polarizers limit the amount of emitted light that actually reaches the photoreactive medium.
Example of Optical Film Use with LCD Display
Referring to FIG. 2, there is shown an example in which different types of optical film are used with LCD devices. In FIG. 2, a typical liquid crystal display 100 comprises a front polarizer 110a, a rear polarizer 110b, a front compensation film 120a, a rear compensation film 120b, and two liquid crystal cell substrates 130a and 130b that sandwich a switchable liquid crystal layer 140. A back light 150 provides source illumination through liquid crystal display 100 for display to a viewer 160. As is well known in the optical arts, liquid crystal display 100 may alternately include additional optical films, such as diffusive or light enhancement films, or may exclude some of the components shown in FIG. 2, such as front and rear polarizers 110a and 110b or front and rear compensation films 120a and 120b. As is also well known in the optical arts, front polarizer 110a, typically a linear polarizer, comprises a polarizing layer 112a, which can be stretched polyvinyl alcohol (PVA) film for polarizing light, between protective layers 111a and 113a, which are typically triacetate cellulose (TAC) films. Preferably, protective layer 111a, on the outer edge of liquid crystal display 100, is treated for low ultraviolet (UV) light transmittance in order to shield polarizing layer 112a from ambient UV light, such as from room lighting or sunlight. Front compensation film 120a may also include a TAC layer as a substrate 121a and an optically anisotropic layer 122a. Substrate layer 121a is then adjacent to protective layer 113a of front polarizer 110a. Alternately, substrate layer 121a could be combined with protective layer 113a as a single protective or substrate layer. Rear polarizer 110b is similar in structure to front polarizer 110a, having a polarizing layer 112b between protective layers 111b and 113b, which are typically triacetate cellulose (TAC) films. Likewise, rear compensation film 120b comprises a substrate layer 121b and an anisotropic layer 122b. Unlike protective layer 111a, however, the TAC film at substrate layer 121b is not normally selected or treated for low UV transmittance.
Prior Art Photoalignment Methods
Among proposed prior art solutions for photo-alignment are a number of scanning solutions, such as the following:                U.S. Pat. No. 5,889,571 (Kim et al.) discloses an irradiation device for scanning linearly polarized UV across a substrate to achieve alignment layer uniformity. U.S. Pat. No. 5,889,571 emphasizes the importance of oblique radiation. This solution is best suited to a substrate provided in sheet form rather than to a substrate continuously fed from a web.        U.S. Pat. No. 6,295,110 (Ohe et al.) discloses a laser light-based system for applying polarized UV radiation across a substrate having a photoreactive layer. Designed for substrates having a diagonal in the range of about 10 inches or slightly larger, U.S. Pat. No. 6,295,100 solution provides two-dimensional irradiation over an area that exceeds the size limit for the type of optical radiation employed. However, there are practical limitations in scaling this type of solution to suit a web-fed substrate having a width dimension of 1 m or larger.        
It has been noted that high irradiance conditions would be beneficial for use in high-speed roll-to-roll manufacturing apparatus, particularly where it is desirable to provide a compact system. Peak irradiance on the web in such environments could range from approximately 50 milliwatts/cm2 to values of several hundred milliwatts/cm2. This means that average irradiance on any polarizer would be much higher. With irradiance over ranges such as might be supplied using a medium pressure long-arc Mercury lamp at power levels in the 100-600 W range, conventional, resin-based polarizers would not be well-suited. For example, this type of irradiation exceeds the practical working range of conventional polarizers such as the HNP′B—Linear Polarizer from 3M (St. Paul, Minn.). Sheet polarizers are not generally capable of handling higher irradiation levels and may quickly deteriorate over a prolonged exposure period. With this limitation in mind, prior art solutions for providing polarized irradiation over a large area include the following:                U.S. Pat. No. 6,190,016 (Suzuki et al.) discloses an irradiation apparatus using an oval focusing mirror, integrator lens, and polarizer disposed at various points in the optical system. U.S. Pat. No. 6,190,016 emphasizes the value of collimated light, incident to a polarizer, to improve polarization performance. The use of Brewster plate polarizers for large scale surfaces is disclosed.        U.S. Pat. No. 5,934,780 (Tanaka) discloses an exposure apparatus using a UV light source having an oval focusing mirror, where the apparatus includes an integrator lens, polarizer, and collimation optics. Brewster plate polarizers are used in the preferred embodiment. This type of solution may work well for a substrate up to a certain size. However, there are practical size limitations that constrain the use of Brewster plate polarizers for large substrates. Similarly, EP 1 020 739 A2 (Suzuki et al.) discloses a modified Brewster plate arrangement. As a variation on Brewster plate polarizers, EP 1 172 684 (Suzuki et al.) discloses a modified V-shaped Brewster's angle arrangement. However, similar weight and size constraints also limit the feasibility of this type of solution.        U.S. Pat. No. 6,292,296 (Choi et al.) discloses a large scale polarizer comprising a plurality of quartz segments disposed at Brewster's angle, used for a system that irradiates a photoreactive substrate using UV. However, such an arrangement would be very costly and bulky, particularly as a solution for a web-fed exposure system with a large irradiation area.        
As the above-noted patent disclosures show, irradiation apparatus designed for large exposure zones have employed sizable polarization components, typically quartz or glass plates disposed at Brewster's angle. Hampered by the relative size and weight of these polarizers, such irradiation apparatus are necessarily less efficient in delivering light energy to the exposure surface. Moreover, conventional polarizers using Brewster plates or interference polarizers based on Brewster's angle principles also exhibit a high degree of angular dependency. That is, incident light must be substantially collimated in order to obtain a uniform polarized light output.
Significantly, solutions using Brewster plate polarizers such as those shown in the U.S. Pat. No. 5,934,780 (Tanaka) and U.S. Pat. Nos. 6,061,138 and 6,307,609 (Gibbons et al.) are inefficient with respect to the percentage of light energy that actually reaches the exposure zone. A substantial amount of light energy is dissipated through polarizers, filters, and uniformizing components, where used.
Light Source Options
A number of different types of light sources for photo-alignment have been disclosed, for example:                WO 00/46634 (Schadt et al.) discloses a method for alignment of a substrate using an unpolarized or circularly polarized source, applied in an oblique direction.        U.S. Pat. No. 4,974,941 (Gibbons et al.) discloses alignment and realignment, preferably using a laser source.        U.S. Pat. No. 5,389,698 (Chigrinov et al.) discloses use of linearly polarized UV for photopolymer irradiation. Similarly, U.S. Pat. No. 5,936,691 (Kumar et al.) discloses use of linearly polarized UV for photopolymer irradiation, with the UV source positioned close to the substrate surface.        
By and large, conventional irradiation systems for alignment are characterized by inefficient use of light. As is noted above, loss of light intensity due to filtering, polarization, and optical components, means that only a fraction of the emitted light arrives as exposure energy. For this reason, a sizable increase in light output is required in order to obtain even a modest increase in exposure energy.
Collimation
As noted above, the use of collimated or substantially collimated light is, in large part, dictated by polarizer characteristics. In related exposure processing applications for photoreactive media, collimated light is considered advantageous, as in these examples:                U.S. Pat. No. 5,604,615 and EP 0 684 500 A2 (Iwagoe et al.) disclose forming an alignment layer by directing collimated UV through slits in a photomask.        In a related curing application, U.S. Pat. No. 6,210,644 (Trokhan et al.) discloses directing UV through slatted collimator for curing resin.        U.S. Pat. Nos. 6,061,138 and 6,307,609 (Gibbons et al.) disclose a method and apparatus for alignment using exposure radiation that is “partially polarized” and “partially collimated.” By “partially polarized,” this disclosure identifies a broad range of S:P values from 1:100 to 100:1 with preferred range from 0.5:1 to 30:1. By “partially collimated” these disclosures identify a broad range with a divergence, in one direction, greater than about five degrees and less than about 30 degrees. The use of such broad ranges simply seems to indicate that some significant degree of variability is acceptable for both polarization and collimation. Indeed, in practice, most polarizers work within the broad range stated in U.S. Pat. No. 6,061,138, particularly over sizable exposure zones. As is generally well known and shown in the disclosure of U.S. Pat. No. 6,190,016 (Suzuki et al.), some degree of collimation is needed simply for consistent control of polarization. Partial collimation, over the broad ranges stated in U.S. Pat. No. 6,061,138, occurs when light simply passes through an aperture and is not otherwise blocked, focused, projected, or diffused. Baffles or apertures that block stray light necessarily perform “collimation” within the ranges given in U.S. Pat. No. 6,061,138. Earlier work, disclosed in U.S. Pat. No. 5,934,780 (Tanaka) similarly shows use of partially collimated light having relatively poor polarization and the use of relatively high incident angles for exposure energy, covering the ranges specified in the '138 disclosure. Another earlier patent, EP 0 684 500 A2 (Iwagoe et al.), states that collimation of the irradiating polarized light beam is preferable, but does not require collimation.        
Thus, prior art seems to indicate that collimation, considered by itself, is not as important as other characteristics of exposure radiation. Certainly, some degree of collimation is inherently necessary in order to efficiently collect and direct light onto a substrate, taking advantage of the light emitted in all directions to improve efficiency by using devices such as using reflective hoods, for example. As is noted above, some degree of collimation is necessary for polarizing light, since polarization devices are not typically equipped to handle wide variations in incident light divergence. But, taken in and of itself, collimation may have secondary importance relative to other properties of the exposure light.
Polarization
In contrast, maintaining a consistent polarization direction or azimuthal angle appears to be very important for obtaining good results for photoalignment. The direction of polymerization or selection for LC alignment materials closely corresponds to the direction of polarization. In fact, there is evidence that partial polarization, as suggested by U.S. Pat. No. 6,061,138 and as exhibited in earlier work disclosed by Schadt et al. (Japanese Journal of Applied Physics, Vol. 31, pp. 2155-2164) appears to be acceptable, provided that a consistent direction of polarization is maintained. The disclosure of U.S. Pat. No. 5,934,780 emphasizes the importance of this direction of polarization. It has been shown that optimal results are obtained over the exposure zone when the exposure energy is somewhat uniformly distributed and when the direction of polarization is tightly controlled to within about one degree.
As is shown in the prior art solutions cited above, achieving polarization over a broad exposure zone, with a tightly controlled direction of polarization, is particularly difficult with high intensity UV-B radiation. It is difficult to obtain a UV-B source that provides polarized UV-B light at reasonable cost. Moreover, high heat and irradiance requirements place considerable demands on filtering and polarization components. Conventional resin-based sheet polarizers are unlikely to withstand the elevated irradiance and high heat conditions. Brewster plates and interference filters can withstand heat conditions but have size and weight disadvantages as well as acceptance angle constraints.
Intensity
As a further complication, controlling the intensity of radiation energy has been proven to be difficult to achieve and to maintain as a web of photoreactive material is exposed. While tolerances may not be critical, some reasonable degree of uniformity appears to be desirable.
Cost-effective mass manufacture of photoreactive LC material requires high throughput. This necessitates using sufficient intensity levels, consistently applied to a material that is exposed and cured at fast speeds. Although conventional solutions provide some capability for high-volume web-fed manufacture, there is clearly room for improvement over prior art approaches with respect to light efficiency, processing speed, cost, and quality.
With respect to visible light, most optical films used with LCD displays are transmissive. However, not all transmissive films used for liquid crystal applications are provided on transmissive substrates. For example, optical films for liquid crystal on silicon (LCOS) devices may be fabricated on reflective substrates, as is described by H. Seiberle, D. Muller, G. Marck, and M. Schadt in the article “Photoalignment of LCOS LCDs” in Journal of the SID, 2002, pp. 31-35. For such devices, LPP photoalignment is complicated by the reflective substrate. Here, incident light that impinges on the LPP layer goes through the film and is reflected back up through the LPP layer, with some impact on the tilt angle achieved. For an LPP layer of 50 nm thickness, for example, about 30-40% of the incident light is reflected back through the optical film. Polarization states of reflected and incident light, although in the same plane, differ in direction, which can have the effect of decreasing tilt angle. On the other hand, the increased radiation from reflection may counteract this influence for some materials and have the opposite effect of increasing tilt angle. As the Seiberle et al. article notes, this complicates the task of photoalignment, requiring selection of an LPP material that is suitable for the effects of the reflected light. Different LPP formulations showed different results. With some LPP formulations, reflected light appeared to improve pretilt angle; other formulations showed a decrease in tilt angle with the added exposure effect.
The Seiberle et al. article acknowledges technical difficulties that must be addressed for photoalignment of a photoreactive layer on a reflective substrate and provides approaches for simulating exposure conditions in order to assess the resulting behavior of the photoreactive layer. It is instructive to observe that, using conventional thinking, the reflective effects noted in the Seiberle et al. article present a technical hurdle that must be overcome in order to obtain controlled photoalignment. This is particularly true since, for conventional reflectors, the polarization direction of reflected light differs from that of incident light. Thus, as is shown in the Seiberle et al. paper, the proposed solution is to determine which set of LPP formulations work best under conditions where light is reflected back through the LPP substrate. In terms of controlling and measuring photo-alignment, then, the reflective substrate is a disadvantage, making the task of providing exposure with controlled polarization more complex when fabricating optical films. In conventional thinking, reflective effects complicate optical film fabrication and are best avoided if possible; there is no indication or suggestion to deliberately employ reflective behavior as a beneficial effect in optical film manufacture.
Notably, where reflective materials are actually used in a multilayer film containing an alignment layer, such as the holographic reflector in the identification device disclosed in international patent application WO 99/53349 (Seiberle et al.), reflective properties are not used for fabrication, but are used only to direct visible light through the alignment layer once formed.
While conventional photo-alignment methods provide significant advantages over older rubbing methods, there is considered to be room for improvement. It can be appreciated that there would be benefits to manufacturing apparatus and methods for fabrication of optical film using photo-alignment, where the exposure radiation is used to condition the response of photoreactive materials more efficiently, allowing the use of lower cost light sources that require less power and alleviating heat concerns for components within the manufacturing system.